The amount of information communicated over an optical fiber communication system is increased with the use of optical wavelength division multiplexing. Wavelength Division Multiplexed (WDM) systems employ WDM signals consisting of a number of optical signals at different wavelengths, hereinafter referred to as channels or information carrier signals, to transmit information on optical fiber cables. Each channel is modulated by one or more information signal, resulting in the capability to transmit a significant number of information signals over a single optical fiber cable. It is recognized that although a WDM signal comprises a plurality of wavelengths capable of carrying channels, not all the wavelength must contain a channel.
To facilitate the subtraction and/or addition of particular channels to and/or from the WDM signal at different points within a network, OADMs are employed that consist of a plurality of optical filters. These OADMs are used to selectively extract Channels, hereinafter referred to as drop channels, from a WDM signal while the remaining channels, hereinafter referred to as through channels, travel through. The OADMs can also be used to add channels, hereinafter referred to as add channels, to a WDM signal, using wavelengths that have been vacated as a result of channels being dropped at the OADM in question or at an OADM earlier in the transmission path. Since more than one channel usually needs to be accessed at a network node, multi-channel OADMs are used such that a plurality of channels can be dropped and/or added from and/or to a received WDM signal.
There are a number of different implementations for a multi-channel OADM. One key factor that must be considered when considering different implementations is the cost of the filters that are utilized. Filters increase in cost as their Figure of Merit (FOM) increases, their FOM being a measure of the complexity of the filter. One skilled in the art would understand that the FOM increases as the tolerance of the filter that is used increases. One factor that can cause an increase in the required tolerance is an increase in the ratio of the passband to dead band, described herein below. Hence, when considering the cost of any particular OADM, one must consider the number of filters required and the overall tolerance of those filters.
With reference to FIGS. 1 and 2, well-known implementations for OADMs are now described. Firstly, FIG. 1 depicts an OADM that comprises a separate channel demultiplexer 102 and channel multiplexer 104. For this OADM implementation, the demultiplexer 102 extracts channels from a WDM signal while the multiplexer 104 inserts channels within the WDM signal output from the demultiplexer 102.
The channel demultiplexer 102, in this case with four channels to be dropped, comprises a first alignment block 106, first and second columnating lenses (CL) 108,110, first, second, third and fourth drop filters (DF) 112a,112b,112c,112d and first, second, third and fourth clean-up filters (CF) 114a,114b,114c,114d. The alignment block 106 is utilized to ensure that beams of light being transmitted between the filters and columnating lenses are aligned properly for optimal performance. The first columnating lens 108 receives an input WDM signal S.sub.IN (t) in a form capable of being transmitted on a fiber optic cable, transforms the signal into an extended beam signal, and transmits the extended beam WDM signal in the direction of the first drop filter 112a. In the example being shown in FIG. 1, the drop filter 112a receives the extended beam WDM signal, filters out a channel at wavelength .lambda.1 with the use of a single wavelength filter, and forwards the remainder of the extended beam WDM signal onto the next drop filter 112b. The isolation of the channel at wavelength .lambda.1 is not perfect and so an additional filter may be required to ensure that only the required channel is sent on for further processing, in this case this is done with the first clean-up filter 114a. Additional pairings of drop and clean-up filters proceed within the channel demultiplexer 102 of the OADM, each operating similar to that of the first drop and clean-up filters 112a,114a but for different wavelengths (.lambda.2,.lambda.3,.lambda.4). In the case depicted in FIG. 1, after four drop filters the resulting extended beam WDM signal is received by the second columnating lens 110 which converts the signal to a form transmittable over fiber optic cable. The signal output from the second columnating lens 110, although not carrying the channels that were dropped within the demultiplexer 102 of the OADM, still may contain channels at other wavelengths.
The channel multiplexer 104 of the OADM of FIG. 1 comprises a second alignment block 116, third and fourth columnating lenses 118,120 and first, second, third and fourth add filters 122a,122b,122c,122d. The second alignment block 116 operates in a similar manner to the first alignment block 106, as do the third and fourth columnating lenses 118,120 with respect to the first and second columnating lenses 108,110 respectively. Each of the add filters 122a,122b,122c,122d are single wavelength filters that insert channels at the wavelengths .lambda.1, .lambda.2, .lambda.3, and .lambda.4 respectively. An output WDM signal S.sub.OUT (t) that is similar to the input WDM signal S.sub.IN (t) but with different channels at wavelengths .lambda.1 through .lambda.4 is transmitted from the fourth columnating lens 120.
With the separate channel demultiplexer and multiplexer 102,104, a large number of filters are required. For each of the drop channels, two single wavelength filters are required, while for each of the add channels, one single wavelength filter is needed. For the case shown in FIG. 1 with four channels being dropped and then subsequently added, twelve single wavelength filters are used. In general, it can be seen that for N channels being dropped and added, 3N single wavelength filters are required. In addition, if higher through isolation is required, an additional single wavelength filter per wavelength would be required. This would bring the total to 4N single wavelength filters required for this design.
There are other well-known implementations for the OADM depicted within FIG. 1 that would result in a similar number of filters being required. One such implementation does not utilize the alignment blocks 106,116, but rather has columnating lenses on either side of each filter element. Instead of the WDM signal being transported from filter to filter within extended beam format, fiber optic cable is used.
Another well-known implementation for an OADM is a band OADM as depicted within FIG. 2. In this implementation, an input WDM signal S.sub.IN (t) is received at a first band filter 202 which extracts a band of wavelengths, in this case wavelengths .lambda.1 through .lambda.4, and passes a WDM signal comprising the remaining channels to an isolation filter 204. The isolation filter 204 ensures that no channels are at wavelengths .lambda.1 through .lambda.4 without extracting or inserting any channels at other wavelengths. Subsequently, the isolation filter 204 outputs the resulting WDM signal to a second band filter 206 at which point a band of wavelengths, in this case .lambda.1 through .lambda.4, are inserted, generating an output WDM signal S.sub.OUT (t)
The channels that are extracted at the first band filter 202 are separated by a series of single wavelength drop filters 208a,208b,208c,208d. These individual channels are then further filtered with respective clean-up filters 212a,212b,212c,212d and output for further processing. As depicted in FIG. 2, the channels inserted at the second band filter 206 are combined prior to the insertion with the use of a series of single wavelength add filters 212a,212b,212c,212d.
Similar to the OADM of FIG. 1, the band OADM of FIG. 2 has a large number of required filters. As can be seen for the case within FIG. 2, twelve single wavelength filters are required for the drop, clean-up, and add filters along with three four wavelength filters for the first and second band filters 202,206 and the isolation filter 204. In general, it can be seen for the case of a band of N wavelengths being dropped and added, there would be 3N single wavelength filters and three N wavelength filters required.
There are some network situations in which only a small number of filters are required in any one OADM. One such well-known network is depicted in FIG. 3. This network comprises a hub 302 and first, second, and third network nodes 304,306,308. As depicted in FIG. 3, the hub 302 communicates independently with each of the nodes 304,306,308 through the fiber optic cables 310,312,314,316. These cables are connected up within a ring configuration that has all transmissions in one direction. In this case, the hub 302 comprises a multi-channel OADM capable of dropping and adding the channels at wavelengths .lambda.1, .lambda.2, and .lambda.3. The nodes 304,306,308 each comprise a single wavelength OADM for dropping and adding channels at respective wavelengths .lambda.1, .lambda.2, and .lambda.3. Hence, it can be seen that the hub 302 communicates with the first node 304 via the channel at wavelength .lambda.1, the second node 306 via the channel at wavelength .lambda.2, and the third node 308 via the channel at wavelength .lambda.3.
The key difficulty with the network configuration as depicted within FIG. 3 is the cost of the OADM within each of the network nodes. Since the same wavelength that is being dropped is also being added at the same node, the tolerance of the single wavelength filters within the OADMs must be extremely high to prevent cross-talk problems. Alternatively, isolation filters are required.
Since the cross-talk possibility is extremely high in this configuration, a typical solution is to use a first set of channels for transmission from the hub 302 to the nodes 304,306,308 and another set of channels for transmission from the nodes to the hub. For instance, this could be done by using wavelengths .lambda.1, .lambda.2, and .lambda.3 for transmitting data to the respective network nodes 304,306,308, similar to that shown in FIG. 3, and using wavelengths .lambda.4, .lambda.5, and .lambda.6 for transmitting data to the hub 302 from the respective nodes 304,306,308. The key problem with this is that the bandwidth efficiency of the network becomes only 50% as at any one time only half the channels are being used.
A further solution to improve the cost of OADMs is not to insert channels within adjacent wavelengths. Such unused wavelengths, commonly called dead bands, allow a relaxation of the tolerances required for the filters used by decreasing the ratio of passband to dead band. Unfortunately, at the same time as reducing the cost of the filters used by reducing the tolerances needed, the bandwidth efficiency of the overall network is significantly reduced. For every used wavelength there is an unused wavelength, making the efficiency 50%. In situations where the bandwidth of the network is critical such a low bandwidth efficiency is not acceptable.
OADM designs and network configurations of OADMs are required that reduce the overall cost of the network while not limiting the bandwidth efficiency. To accomplish this, the number of filters and the tolerance of the filters must be reduced without significantly sacrificing the limited bandwidth of the network.